Parameters important for the suitability of a film for the packaging of perishable goods are the oxygen barrier and also, in the case of transparent packaging, the permeability of the film to UV (the ultraviolet portion of light). The second item is particularly important in the case of foods, for example those presented in supermarket counters which are subject to irradiation by lamps with relatively high UV emission. If the UV permeability of the packaging here is too high the result can be unattractive discoloration and changes in taste.
A particularly important third criterion for the suitability of a packaging film is its price, since even relatively small price differences can shift the purchasing decision in the direction of a more favored product, even if the packaging does not meet high quality requirements with regard to durability.
The polyester-based packaging films currently available in the market are almost exclusively PET—(polyethylene terephthalate)-based films, such as Hostaphan® RNK (Mitsubishi Polyester Film GmbH, Wiesbaden). Although these films are low-priced and generally have an acceptable oxygen barrier for many applications, this barrier is nevertheless often inadequate, in particular for transparent, unmetallized packaging. In addition, PET retains relatively high permeability to UV light.
Films made from PEN (polyethylene naphthalate) provide a substantially higher oxygen barrier and also have lower UV permeability, but are uneconomic due to the high price of PEN and are therefore little used in industry.
Although this problem can be reduced to some extent by using multilayer films which comprise one or more PEN layers on a PET base (e.g. DE-A-197 20 505 or WO 01/96114), these films have problems with delamination of the layers made from the different polyesters and are relatively difficult to produce. In addition, the regrind arising cannot subsequently be reused for a product of the same value, since PET and PEN become mixed during the regrinding process and can therefore no longer be utilized to produce the high-value layers comprising 100%, or almost 100% by weight, of PEN. The material can only replace the comparatively inexpensive PET. For these reasons, although these films are less expensive than films composed mainly of PEN they are considerably more expensive than standard PET packaging films, and are therefore used only in niche markets with particular requirements.
Films made from blends of PET and PEN have also been described (e.g. GB 2 344 596), but here there is no indication as to how these blends can be used to produce a film with improved UV barrier and oxygen barrier, capable of being used to produce transparent packaging with increased shelf life. Nor are there any indications of the quantitative PET/PEN ratios suitable for producing films with good oxygen barrier and UV barrier.
Another very general method for improving the UV permeability of polyester films is to add UV stabilizers/absorbers e.g. Tinuvin® (Ciba Speciality Chemicals, Switzerland). However, the use of these compounds is likewise accompanied by relatively high costs, and, depending on the stabilizer used, creates problems with suitability for direct contact with foods.
Packaging made from PEN copolymers or from PET/PEN copolymers (or blends) also has very high blue fluorescence intensity when it is irradiated with UV light. In the case of PET/PEN copolymers with low PEN content of <5%, this intensity is indeed even higher than for straight PEN. While slight blue fluorescence can indeed be desirable to eliminate a yellow tinge in the film, strongly clearly visible blue fluorescence makes the contents appear unnatural (artificial), and this can adversely affect the consumer's purchasing decision. This fluorescence should therefore be minimized.
An object of the present invention was then to provide a film which, when compared with straight PET films, provides a better oxygen barrier and UV barrier, can be produced at low cost and is suitable for packaging applications, and, when compared with packaging comprising PET films, ensures an improved lifetime for the packaged goods, even at relatively high UV doses. In addition, the film or the packaging produced therefrom should not have any tendency toward particularly strong blue fluorescence when irradiated with UV light.